This invention relates to semiconductor wafer polishers for polishing faces of semiconductor wafers.
The final step in a conventional semiconductor wafer shaping process is a polishing step to produce a highly reflective and damage-free surface on one face of the semiconductor wafer. Polishing of the semiconductor wafer is accomplished by a mechanochemical process in which a rotating polishing pad rubs a polishing slurry against the wafer. In a conventional semiconductor wafer polisher, the wafer is bonded with wax to a polishing block and then held against the rotating polishing pad by a polishing arm.
Semiconductor wafers must be polished particularly flat in preparation for printing circuits on the wafers by an electron beam-lithographic or photolithographic process. Flatness of the wafer surface on which circuits are to be printed is critical in order to maintain resolution of the lines, which can be as thin as 1 micron or less.
Flatness is quantified in part by a Total Thickness Variation measurement (TTV) and Site Total Indicated Reading (STIR). TTV is the difference between the maximum and minimum thicknesses of the wafer. STIR is the sum of the maximum positive and negative deviations of the surface in a small area of the wafer from a reference plane which is parallel to the back surface of the wafer and intersects the front surface at the center of the local site. Total thickness variation in the wafer is a critical indicator of the quality of the polish of the wafer. Preferably, a polished semiconductor wafer will have a TTV of less than one micron and a STIR of less than one-half micron for any 20 mm.times.20 mm local site.
In conventional wafer polishing some of the frictional heat generated by the rubbing action of the wafer, polishing pad and slurry is transferred to the polishing block and the polishing turntable. This heat transfer induces temperature gradients through the polishing block and turntable which cause thermal expansion of the polishing block and turntable. The thermal expansion adversely affects the flatness of the polishing block and turntable and therefore adversely affects the flatness of polished wafers, particularly if the thermal expansion is uncontrolled and varies from polishing cycle to polishing cycle. For example, in the idle time between polishing cycles, the induced distortion in the turntable begins to dissipate as its temperature equalizes. If this idle time varies, successively polished wafers may show significant flatness variations.